The application of wireless communication technology to centralized distribution of information increasingly demands that radio equipment such as transceivers and antennas be specifically adapted to the particular end use. In one application of wireless technology to cargo tracking systems, two-way paging systems are used for sending and receiving digital signals at low data rates to and from individual cargo containers. The two-way paging network may use satellite and/or terrestrial radio links to communicate with miniature paging transceivers located in each container.
Such paging equipment may operate in the High Frequency (HF) and/or Very High Frequency (VHF) radio band. At those frequencies, most antennas are of the monopole or so-called whip type and are electrically short, being of a physical length of one-quarter wave length or less. For example, whips intended to operate in the HF band are typically of a length of approximately 96 inches or more.
Unfortunately, whip antennas are not convenient for adaptation to most types of cargo containers directly. This is true for several reasons. The containers may be of the type adapted for use as a trailer for a truck, a railway boxcar, or a shipping container adapted for cargo ships which is to be stacked in a shipyard and moved via trailers and/or railway cars.
When such containers are mounted on trailers or trucks, for example, state and Federal highway regulations typically require clearance for bridges and underpasses to be thirteen feet and six inches (13'6"). Many trailers already are designed to be of a maximum safe clearance height such that there is close tolerance on the clearance. It is therefore not practical to attach an additional 30" vertical whip antenna.
In other applications, it is desirable to stack containers such as in a shipyard or on a cargo ship. It is not practical in those instances for antennas to be protruding from the sides, upper or lower surfaces of the containers, as they would therefore be deformed or damaged.
What is needed is an antenna for use with metallic containers and other large terrestrial vehicles that avoids these difficulties.